The invention relates to a method for connecting a first electrical storage device and a second electrical storage device in parallel, the first electrical storage device and the second electrical storage device in each case having a no-load voltage UBat1, UBat2 and an impedance R1 and R2, respectively, wherein the first electrical storage device is connected to connecting terminals for charge withdrawal or charge supply via two first electrical conductors, wherein the electrical connection between the first storage device and one of the connecting terminals can be established or interrupted by means of a first switch, wherein the second electrical storage device is connected to the connecting terminals via two second electrical conductors, wherein the electrical connection between the second storage device and one of the connecting terminals can be established or interrupted by means of a second switch and wherein the no-load voltage UBat1 of the first storage device is greater than the no-load voltage UBat2 of the second storage device. Furthermore, the invention relates to a circuit of a first electrical storage device and a second electrical storage device, wherein the first electrical storage device is connected to connecting terminals for charge withdrawal or charge supply via two first electrical conductors, wherein a first switch is provided in one of the first electrical conductors, which interrupts or establishes the electrical connection between the first storage device and the connecting terminals, wherein the second electrical storage device is connected to the connecting terminals via two second electrical conductors, wherein a second switch is provided in one of the second electrical conductors, which interrupts or establishes the electrical connection between the second storage device and the connecting terminals.
The usability of non-stationary electrical loads such as, for example, electric drives, depends essentially on the capacity of the usable storage device for electrical energy. Typical energy storage devices for non-stationary applications are electric batteries, accumulators or capacitors.
It is characteristic of this class of energy storage devices that the state of charge in the unloaded state is correlated with a particular terminal voltage. Given the same storage technology and same state of charge, the volume of the storage device is linearly correlated with the stored quantity of energy, that is to say, the larger the storage device the greater the energy stored in it.
Apart from cost aspects, the maximum energy content of a storage device also has mechanical aspects. Under certain circumstances, certain dimensions must not be exceeded in order to provide for the installation, e.g., in a vehicle, and the transportation of the storage device. Furthermore, limits are also set to the total mass by requirements derived from the handling and the transportation.
If the total energy requirement exceeds the limits of a storage device which are derived from the dimensions and the mass, several storage devices must be connected together. The necessity of connecting several storage devices together to form a large storage device can also be derived from the available installation space.
If the storage device is a battery, the latter is constructed of basic units, the battery cells. The energy content of a battery cell depends on the cell technology and the cell volume. By suitably selecting the number of cells and their interconnection to form so-called core packs, the energy content and the current carrying capability of the core pack can be arranged to be proportional to the number of cells.
Furthermore, it is possible to adapt the impedance of the battery to the impedance of the load by means of a suitable ratio of series connection to parallel connection of the core packs within a battery, i.e., the battery can be adjusted in such a manner that it is capable of delivering also the maximum current needed with the required energy content.
The energy content of the storage device can be increased by connecting several batteries in series and/or in parallel. In the case of a series connection of a number of batteries, however, the increased system voltage results in higher demands on the insulation and on the creepage distances and air gaps to which the design of the individual battery frequently does not correspond since its insulation system is exclusively adjusted to the voltage level of individual operation.
With a limited dielectric strength of the individual battery or of the loads to be connected, connecting several batteries in parallel is therefore considered as being the more suitable measure for increasing the energy content of the storage device. This also applies to storage devices consisting of accumulators or capacitors.
To meet various safety requirements, electrical storage devices are equipped with a battery switch. If the switch is deactivated, the accessible battery terminals are free of voltage.
When electrical storage devices are connected in parallel, terminals of identical polarity are connected to one another. This is unproblematic as long as at least N−1 battery switches are opened with N storage devices.
In the time in which the storage devices are not connected to the common bus bar, their voltage level can develop differently. Causes of this can be, for example, separate charging of different periods of time or also the different self-discharge when the storage device is not used over a relatively long period of time.
If the voltages of the storage devices are different, a current will flow from the storage device having the higher voltage to the storage device having the lower voltage after two storage devices are connected together, that is to say after the corresponding two battery switches have been closed, which current produces a charge equalization until the voltages are equal over both storage devices. This applies with the assumption that no load was connected to the parallel-connected storage devices across which the voltage is lower than the no-load voltage of the storage device having the lower charge. The magnitude of the current flow is defined by the voltage difference between the two storage devices and the series resistance. In the case where two identical storage devices are connected in parallel, double the internal resistance of one storage device is effective with respect to the equalizing current.
In the case of N storage devices connected in parallel, the equalizing current leads to turn-on losses at N−1 battery switches if the voltage of the battery to be connected differs from the voltage of the battery already connected or from the resultant voltage of the batteries already connected, respectively. When the switch of the first storage device is closed, no current flow takes place as yet since the difference voltage is still dropped across the second switch. If then the switch of the second storage device is closed, a powerless peak resulting as the product of the voltage drop across the switch and the effective equalizing current is produced until the steady-state forward resistance of the switch is reached. Due to the switching power loss occurring, the switching contacts are eroded. The switches must therefore be specified for a maximum current to be switched and for the character of the load. The equalizing current produces losses across the effective equivalent resistance of the storage devices so that a part of the stored energy is converted into heat during the process of charge equalization and thus of voltage equalization.
If two lithium manganese batteries have, for example, an internal resistance of 1 mOhm each and when these batteries are connected in parallel, their battery voltages differ by, for example, 10 volts, and therefore an equalizing current of 5000 A which can represent a high hazard potential, flows due to the very low internal resistance.
From EP 1811592 B1, a battery having a safety circuit is known which has a resistor in the connection between the battery cells and one of the battery poles and where a switch is arranged in parallel with the resistor, and where the switch is closed only when the current flowing between the battery cells and one of the battery poles drops below a predetermined minimum current.
It is the object of the present invention to propose a method for connecting two or more electrical storage devices in parallel, and a corresponding circuit which enables storage devices of different no-load voltage to be connected in parallel with a minimized loading of the switches involved, wherein the charge equalization between the storage devices is minimized or completely prevented. Equalizing currents flowing when two storage devices are connected together should be avoided or limited to a minimum, if possible.
It is a further object to present a method and a circuit which allow two electrical storage devices to be connected in parallel without or with only little interruption of the power withdrawal.
All, or at least a part of these objects are achieved by means of a method of the type mentioned initially, which is characterized in that during a charge withdrawal, the electrical connection between the first storage device and the connecting terminals is established by means of the first switch and the electrical connection between the second storage device and the connecting terminals is interrupted by means of the second switch so that initially charge is withdrawn only from the first storage device, and in that the electrical connection between the second storage device and the connecting terminals is established by means of the second switch when the difference between the voltage dropped across the connecting terminals of the first storage device and the no-load voltage of the second storage device is less than a predetermined differential add-on voltage.
A circuit according to the invention of a first electrical storage device and of a second electrical storage device, wherein the first electrical storage device is connected to connecting terminals for charge withdrawal or charge supply via two first electrical conductors, wherein a first switch is provided in one of the first electrical conductors which interrupts or establishes the electrical connection between the first storage device and the connecting terminals, wherein the second electrical storage device is connected to the connecting terminals via two second electrical conductors, wherein a second switch is provided in one of the second electrical conductors which interrupts or establishes the electrical connection between the second storage device and the connecting terminals, is characterized in that a first measuring instrument for detecting the voltage present on the battery side of the first switch between the two first electrical conductors and a second measuring instrument for detecting the voltage present on the battery side of the second switch between the two second electrical conductors are provided, wherein the first and the second measuring instrument are connected to a control unit which has a comparing unit for comparing the detected voltages and a drive of the first and/or second switch in dependence on the comparison.
Exactly the opposite is preferably carried out in the case of a charge supply: in the case of a charge supply, the electrical connection between the second storage device and the connecting terminals is first established by means of the second switch and the electrical connection between the first storage device and the connecting terminals is interrupted by means of the first switch so that initially only the second storage device is supplied with a charge. The electrical connection between the first storage device and the connecting terminals is established by means of the first switch only when the difference of the voltage dropped across the connecting terminals of the second storage device and the no-load voltage of the first storage device is less than a predetermined differential turn-off voltage.
By means of the measuring instruments, the instantaneous terminal voltage of the storage device or storage devices which are already connected to the load and the no-load voltage of the storage device to be added next can be detected. These values are compared with one another in the control unit. When these two values meet a predetermined condition, especially when they differ from one another by no more than a predetermined differential add-on voltage or differential turn-off voltage, the control unit drives the switch of the storage device to be added or to be turned off in order to close it or to open it. The circuit according to the invention thus allows the first or second storage device to be connected in parallel to form a common storage device or the first or second storage device to be decoupled from an existing parallel circuit in dependence on the terminal voltage and the no-load voltage of the first or second storage device.
In the text which follows, the term storage device means a storage device for storing electrical energy. In particular, a storage device is understood to be a battery, an accumulator or a capacitor. In accordance with the general linguistic usage, the term battery is also intended to comprise accumulators and rechargeable batteries within the context of the present application. The invention is used with particular advantage to connect lithium-ion-accumulators, particularly lithium-manganese accumulators in parallel since these batteries or accumulator types have a low internal resistance so that without use of the invention in connecting these batteries/accumulators in parallel, high currents could flow. In particular, the invention is used for connecting in parallel storage devices, particularly batteries or accumulators, having the same nominal voltage.
The term connecting terminals is understood to mean the connections of the storage device via which charge can be withdrawn from or supplied to the storage device. The term terminal voltage characterizes the voltage present at these connecting terminals.
According to the invention, a switch is provided in one of the electrical connecting lines between the storage device and the connecting terminals. When the switch is opened, that is to say the electrical connection between the storage device and one of the connecting terminals is interrupted, the voltage dropped between the two electrical connecting lines on the battery side of the switch, that is to say the voltage dropped between the part-piece connecting the storage device and the switch of one electrical connecting line and the other electrical connecting line corresponds to the no-load voltage of the storage device. When the switch is closed, this voltage corresponds to the terminal voltage. Terminal voltage and no-load voltage can thus be determined by means of the same measuring instrument, the switch being closed in the first case and the switch being opened in the second case.
The switch establishes the electrical connection between the associated storage device and its connecting terminals or interrupts them. When the switch is opened, the storage device and the connecting terminal connected to the connecting line with the switch are electrically decoupled from one another, i.e. there is no current-conducting connection. There is no electrical connection between the storage device and the corresponding connecting terminal in parallel with the electrical connecting line in which the switch is arranged, either. When the switch is opened, the connection between the storage device and the connecting terminal is therefore completely interrupted. In particular, no diode arranged in parallel or anti-parallel with the switch such as, for example, a parasitic diode or a resistor are provided via which current could flow under certain conditions.
It is essential to the invention that with a charge withdrawal, the adaptation of the voltages of the individual storage devices is brought about by loading the first storage device, i.e., the storage device first connected to the load. Conversely, during loading of the storage devices, the voltages of the individual storage devices are adapted by charging the second storage device which is connected first to the source in this case. The voltage adaptation is carried out not via a precharging resistor which, for example, could be connected between the first and the second storage device but by loading or charging the storage device connected first to the sink or source. According to the invention, neither precharging resistors nor diodes or similar components are provided in parallel with the switch. The storage device is either connected directly to the connecting terminals (with the switch closed) or separated completely from the connecting terminals (with the switch opened). In this manner, the power loss is kept as low as possible when storage devices are connected in parallel.
The electrical resistance of a storage device is called its impedance and impedance can have both dynamic and static components. In steady-state operation, the impedance corresponds to the internal resistance of the storage device. In this context, steady-state operation means that a constant current is delivered over several milliseconds and the change in temperature of the storage device, occurring within this time, is negligible.
The term load is understood to be an electrical load, particularly an electric motor which is supplied with electrical energy from the storage device or devices. In the wider sense, however, it can also be a load which does not consume any current but which supplies charge to the storage device or devices. If the latter meaning is meant, this will be expressly pointed out in the text which follows.
According to the invention, the storage devices are added in dependence on their no-load voltages. During a charge withdrawal, charge is initially withdrawn only from the first storage device having the higher no-load voltage. Essentially, the terminal voltage of the first storage device already added is adapted to the no-load voltage of the second storage device by means of the voltage drop across the internal resistance of the first storage device added first. In the case where the load current is not high enough for effecting an adequate voltage drop, the no-load voltages are adapted with time by discharging the first storage device.
In practice, this circuit is preferably implemented by providing a switch and a voltage measuring instrument for each storage device. The switch is arranged in one of the electrical connections between the storage device and one of the connecting terminals of the storage device. The voltage measuring instrument measures the voltage between the battery side of the switch, i.e. between the part-piece which connects the storage device and the associated switch, and the other connecting terminal of the storage device which is connected directly to the storage device without interposition of a switch. When the switch is opened and the corresponding storage device is thus separated from the connecting terminals or from one of the connecting terminals, respectively, the voltage measuring instrument measures the no-load voltage of the storage device. This is ensured by the fact that no further electrical connecting line is provided in parallel with the electrical connecting line having the switch. When the switch is closed, the voltage measuring instrument measures the terminal voltage of the storage device. When no load is connected to the connecting terminals, no-load voltage and terminal voltage are matched.
The differential add-on voltages during the discharging process and the differential add-on voltages during the charging process can be selected independently of one another. Both values can be both positive, negative and zero. The more the differential add-on voltage approaches the value of zero, the further the equalizing current also converges towards zero and the less is the load on the switch which implements the parallel connection. During discharging, the second storage device is connected in parallel in the case of a positive differential add-on voltage when the external terminal voltage, i.e. the voltage between the terminals to which the load is connected has fallen to a value which corresponds to the no-load voltage of the second storage device plus the amount of the differential add-on voltage. Conversely, in the case of a negative differential add-on voltage, the second storage device is connected in parallel when the external terminal voltage has fallen to a value which corresponds to the no-load voltage of the second storage device minus the amount of the differential add-on voltage. This correspondingly applies to the differential add-on voltage selected for the charging process.
Connecting two electrical storage devices in parallel is optimal when the voltage difference is zero or at least minimum at the time of paralleling. The smaller the voltage difference between the energy storage devices, the smaller the voltage drop across the switch closing last and the lower also the equalizing current to be expected so that the reduction in the voltage difference has a quadratic effect with respect to the reduction of the switch losses and thus also with respect to the reduction of the switch wear.
According to the invention, when two electrical storage devices are connected in parallel, the storage device having the higher no-load voltage is first connected to the electrical load.
The no-load voltage of a storage device can be determined with the load not connected to the storage device, for example by measuring the terminal voltage present at the connecting terminals of the storage device. Since, without a load, no current flows, there is no voltage drop across the internal resistance or across the impedance of the storage device, respectively, so that the terminal voltage is equal to the no-load voltage of the storage device.
If the storage device is connected to a load, the no-load voltage can be determined from the terminal voltage and the voltage dropped across the internal resistance of the storage device. The latter voltage drop can be calculated from the instantaneous current flow, for example, when the internal resistance is known. In a preferred embodiment, therefore, a current measuring instrument is provided for detecting the current flowing from or to the first and/or second storage device.
After the load is connected to the first storage device and possibly after the load has been turned on, charge is withdrawn from the first storage device. Depending on the power absorbed and the resultant current, a voltage drop occurs across the impedance of the first storage device by which the terminal voltage is reduced compared with the no-load voltage of the first storage device. In this phase, the load is exclusively supplied with electrical energy via the first storage device.
Apart from the fact that the terminal voltage of the first storage device approaches the no-load voltage of the second storage device due to the voltage drop across the internal resistance of the first storage device, it is also achieved that the first storage device which supplies the current is discharged so that the no-load voltages of the first and of the second storage device become matched with time.
If the first storage device is sufficiently highly loaded, i.e., sufficient current is withdrawn from the first storage device, the voltage dropped across the connecting terminals of the first storage device can approach the no-load voltage of the second storage device by less than a predetermined differential add-on voltage.
When the terminal voltage of the first storage device and the no-load voltage of the second storage device have become matched apart from the differential add-on voltage, the second storage device can be connected in parallel with the first storage device with only little power loss and without greater loading of the switches so that the load is consequently supplied with energy from the first and the second storage device.
A maximum permissible current may be defined for the storage devices. If this current is exceeded, there is a risk that the storage device is damaged. A controller is therefore advantageously provided which ensures that the current drawn from the storage devices does not exceed the maximum current of the respective storage device. This is of significance particularly in the first phase in which the current is exclusively delivered by the first storage device. It is therefore advantageously monitored that the current does not exceed the maximum current of the connected storage devices, particularly of the first storage device.
The second storage device is connected in parallel with the first storage device by closing a switch. Since the switch needs a particular time for the closing process, it is advantageous if the current drawn from the first storage device, at which the voltage drop across the internal resistance of the first storage device has the desired value, is kept constant during the switching process. Since the time needed for this is within a range of milliseconds, a corresponding delay in the drive train of a vehicle is not noticeable, for example when an electrical drive system is supplied.
When the load current is increased further, a charge is withdrawn also from the second storage device. Assuming identical storage devices having identical temperatures and identical recovery state, the current from the first storage device will increase by the same amount as the current from the second storage device. However, both storage devices are still loaded by different amounts so that the first storage device is discharged more than the second storage device and the no-load voltages of both storage devices become matched.
When the load power is reduced, the current drops and the terminal voltage, i.e., the voltage between the connecting terminals to which the load is connected, increases. When the terminal voltage becomes greater again than the no-load voltage of the second storage device, it becomes both possible to retain the parallel connection of the first and second storage device and to turn off the second storage device, that is to say to interrupt the current-conducting connection between the external connecting terminals to which the load is connected, and the second storage device. The second storage device is preferably turned off when the terminal voltage differs from the no-load voltage of the second storage device by a certain differential turn-off voltage and/or when the second storage device, due to the changing load conditions, now only delivers a negligible contribution or no contribution to feeding the load or when the second storage device even begins to absorb energy.
Since the second storage device is connected to the load in this situation, its no-load voltage cannot be measured directly. The no-load voltage is therefore advantageously determined from the terminal voltage and the voltage dropped across the internal resistance of the storage device. The latter voltage drop can be calculated from the instantaneous current flow, for example when the internal resistance is known.
Another possibility of determining the turn-off time of the second storage device consists in measuring the current from or to the second storage device and turning off the second storage device when this current drops below a predetermined maximum current.
During the turning-off, too, it is advantageous to keep the current constant during the turning-off of the second storage device.
However, turning off a storage device will take place at the earliest when the total current no longer exceeds the permissible current of an individual switch or storage device, respectively. Turning the second storage device off early immediately after the terminal voltage has exceeded the no-load voltage of the second storage device prevents losses which would result from the charge equalization between the storage devices.
The individual storage devices are advantageously added in dependence on voltage and turned off in dependence on current. The second storage device is added when the difference of the voltage dropped across the connecting terminals of the first storage device already connected to the load and the no-load voltage of the second storage device is less than a predetermined differential add-on voltage. Conversely, the turning-off, i.e., separating a storage device from the load, is preferably carried out in dependence on current. When the load is reduced, the second storage device is preferably disconnected, i.e., the associated second switch is opened when the current from or to the second storage device drops below a predetermined value.
In a further preferred embodiment, signals are exchanged with a load circuit which is used for controlling the load which is supplied with a charge by the first and/or second storage device. The signals from the load circuit are at least also used for keeping the current flowing to or from the storage device(s) constant during the adding or turning-off of the first or second storage device. During the switching processes, the load current is kept constant in dependence on these signals and possibly in dependence on other quantities.
To avoid any instability of the system within the range of the switching point, it is advantageous to provide a hysteresis. Adding and disconnecting the storage device are carried out at different voltages. This means that the differential add-on voltage when a storage device is added differs from the differential turn-off voltage when the storage device is turned off.
The first storage device or the second storage device can in each case have a number of storage devices connected in parallel which are or were preferably connected in parallel analogously. That is to say the method according to the invention can also be advantageously used with more than two storage devices. Firstly, it is determined which storage device has the highest no-load voltage. This storage device is considered to be the first storage device in the sense of the invention and is turned on and loaded first. If the terminal voltage of this first storage device deviates by less than the differential add-on voltage from the no-load voltage of the storage device having the second highest no-load voltage, the second storage device is turned on.
When the loading increases further, the moment is awaited at which the common terminal voltage of the first and second storage device deviates from the no-load voltage of the storage device having the third-highest no-load voltage by less than the differential add-on voltage. The storage device having the third-highest no-load voltage is then also connected in parallel. This method can be repeated for any number N of storage devices. The storage devices already connected in parallel can here be considered as first storage device in the sense of the invention to which the storage device to be added next is connected additionally in parallel as second storage device in the sense of the invention.
In the case of several storage devices to be connected in parallel, identical differential add-on voltages are preferably selected in each case. This applies both to the discharging and to the charging process. The N-th storage device is in each case connected in parallel when the terminal voltage of the N−1 storage device which is already connected in parallel differs from the no-load voltage of the N-th storage device by a fixed differential add-on voltage. The differential add-on voltage is here the same independently of which storage device is added. However, it is also possible to select different differential add-on voltages for the (N−1) adding processes since the internal resistance decreases with each storage device connected in parallel so that the equalizing current would increase, the differential voltage remaining the same. The differential add-on voltage is therefore preferred to be indirectly proportional to the number of storage devices already connected in parallel.
The method according to the invention is preferably used also during the charging of the storage devices. For example, the invention allows the simultaneous charging of the first and second storage device (and possibly other storage devices) by means of a high-power charger. For example, the invention is advantageously also used in a recovery of braking energy. The above statements made in conjunction with the discharging of the storage devices apply analogously, the order of adding the storage devices being reversed. During the charging only the storage device having the lowest no-load voltage is first charged. When the terminal voltage of the storage device connected to the charger deviates from the no-load voltage of the storage device having the next-higher no-load voltage by less than a predetermined differential add-on voltage, this storage device is also connected in parallel and both storage devices are charged.
The differential add-on voltage during the charging process can be selected to be exactly as large as the differential add-on voltage during the discharging process. However, it is also possible to provide different differential add-on voltages during charging and discharging in order to meet the requirements of the characteristics of the charging and discharging process in a better manner.
Two electrical storage devices are optimally connected in parallel when the voltage difference is minimal at the time of the paralleling. Advantageously, a differential add-on voltage Vd of:Vd<Imax*R is therefore selected, Imax being the lower value of the maximum permissible current during the adding or turning-off of the second storage device 2 and of the maximum current which may be permissibly withdrawn from the first and/or second storage device 1, 2 and R is the resistance effective after the paralleling. Imax represents the smaller value of the maximum permissible switch current and the maximum permissible battery current.
The value of the differential add-on voltage is particularly advantageously selected as:Vd<0.1*Imax*R. 
In the ideal case, no voltage difference between the terminal voltage of the first storage device and the no-load voltage of the second storage device during discharging or, respectively, between the terminal voltage of the second storage device and the no-load voltage of the first storage device during charging.
The invention is particularly suitable for connecting batteries or accumulators, particularly lithium-ion accumulators, especially lithium-manganese accumulators, in parallel. Lithium-ion accumulators have a very low internal resistance so that when such accumulators are connected in parallel, very large equalizing currents can flow even with low voltage differences. The invention avoids such equalizing currents or limits them, respectively.
The circuit according to the invention is advantageously used in electrical drive systems, particularly electrical boat driving mechanisms.
In the text which follows, the invention and further details of the invention will be explained in greater detail with reference to illustrative embodiments shown diagrammatically in the drawings.